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$ sudo apt install ssh

~/.ssh/config

$ mkdir ~/.ssh
$ chmod 0700 ~/.ssh

~/.ssh/config

HostKeyAlgorithms ssh-rsa

$ ssh remote-user@remote

Warning: the ECDSA host key for ... differs from the key for the IP address '... '

Offending key for IP in /home/user/.ssh/known_hosts:line

/home/username/.ssh/username@host name
/home/username/.ssh/username@host name.pub

$ ssh-keygen -h

Generating public/private rsa key pair.
Enter file in which to save the key (/home/username/.ssh/id_rsa):

/home/username/.ssh/username@host name

Enter passphrase (empty for no passphrase):

Enter same passphrase again:

Your identification has been saved in /home/username/.ssh/username@host name
Your public key has been saved in /home/username/.ssh/username@host name.pub
The key fingerprint is:
XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX:XX username@host name
The key's randomart image is:
+--[ RSA 2048]----+
|            .    |
|       o   o  .  |
|      o = o  . o |
|       B . .  * o|
|        S o    O |
|         o o  . E|
|          o o  o |
|           o  +  |
|            .. . |
+-----------------+

/home/username/.ssh/username@host name

/home/username/.ssh/username@host name.pub

$ touch ~/.ssh/config

IdentityFile ~/.ssh/PRIVATE_KEY_FILE

$ cat ~/.ssh/config
...
IdentityFile ~/.ssh/PRIVATE_KEY_FILE
...

$ sudo apt-get install xclip
$ xclip -sel clip < ~/.ssh/username@host name.pub

I will ignore this because it is an external link. 

 > I do not know what is indicated by the link '#software_reference/howto-create-key-pair'.

local $ ping remote.local

/home/local-user/.ssh/local-user@local.pub

ssh-rsa long list of letters and numbers local-user@local

local $ ssh-copy-id remote-user@remote

local $ ssh remote-user@remote

$ chmod 0700 ~/.ssh
$ chmod 0700 ~/.ssh/*